When NBC televised last year's Super Bowl between the Philadelphia Eagles and New England Patriots, the network's most recognizable sportscaster was nowhere to be seen.

The reason, according to 40-year NBC Sports veteran Bob Costas, goes back to comments he made about the NFL's concussion problems more than two years earlier.

A lengthy report by ESPN's Outside the Lines delves into the details of an increasingly uneasy relationship between Costas and professional football — and how it got him into trouble with his longtime employer.

In December 2015, as the movie "Concussion" was about to be released, NBC's Sunday Night Football broadcast was in Pittsburgh for a game between the Steelers and Indianapolis Colts.